The present invention relates to a resonator-type surface-acoustic-wave filter with reduced insertion loss.
The resonator-type surface-acoustic-wave filters (referred to below as resonator-type SAW filters) considered in the present invention have the well-known ladder circuit configuration. The ladder configuration does not require a matching circuit, and has such further advantages as low loss, high attenuation, and narrow bandwidth, but there is a trade-off between loss and attenuation. If the ladder configuration contains only one stage, or a small number of stages, insertion loss is low but attenuation in the stopbands may be inadequate. With a large number of stages, stopband attenuation improves but insertion loss increases, and the filter becomes large in size.
A known solution to this problem couples one or more additional SAW resonators in series with the ladder circuit to improve the stopband attenuation. The present invention addresses this problem in the upper stopband, so the conventional solution would be to provide an additional SAW resonator with frequency characteristics adapted to improve the attenuation in that stopband. This solution is unsatisfactory in the following respects. First, the additional SAW resonator increases the insertion loss to some extent. Second, impedance matching characteristics are worsened, because the resonant frequency of the additional SAW resonator lies above the passband of the filter. (This is because the antiresonant frequency of the additional SAW resonator must be higher than the antiresonant frequency of the SAW resonator or resonators in the series arm of the filter.) Third, the additional SAW resonator increases the size of the filter.